Permanent magnets composed of cobalt and rare earth alloys have come into prominence in recent years because of the very high energy product of such magnets and because they can maintain a high and constant magnetic flux in the absence of an exciting magnetic field.
In order to prepare cobalt-rare earth magnets having the most desirable properties, it is necessary to exercise great care in the production of such magnets. Cech U.S. Pat. No. 3,748,193 discloses and claims the production of cobalt-rare earth intermetallic alloys by reacting cobalt, samarium oxide and calcium hydride to reduce the samarium oxide and diffuse the cobalt and samarium together to form the alloy. Benz U.S. Pat. Nos. 3,655,463, 3,655,464, 3,695,945 and Benz and Martin U.S. Pat. No. 3,684,593 disclose and claim processes for making sintered cobalt-rare earth magnets.
During the course of producing magnets by the various Benz processes, fine particles of cobalt-rare earth materials are pressed or compacted into a green body. The incorporation into the particles of an internal lubricant or binder will achieve denser green bodies, prolong tool life, and produce better powder flowability during the cavity filling. For this reason it is very desirable that lubricants or binders be present at the time of the pressing or compacting. However, rare earths will readily react chemically with the carbon of the binder material during the subsequent sintering of the green bodies. This reaction detracts considerably from the properties of the final magnetic product. Since sintering is a necessry part of the process for producing satisfactory magnets, it has been customary to omit internal binders or lubricants from the particles themselves and instead rely upon external lubricants or binders to the extent that they can provide improved pressing or compacting. At best, however, external binders or lubricants are inferior to internal binders or lubricants.
Not only does carbon react readily at elevated temperatures with rare earths but also other elements such as hydrogen or nitrogen. The patents cited above all emphasize the importance of using inert atmospheres such as would be provided by argon or helium in the course of sintering or heat treating cobalt-rare earth materials.